Stellar Remnants
Stellar remnants are the end products of a star's life cycle, resulting from the processes that occur as stars exhaust their nuclear fuel. The evolution of stars varies significantly with their mass; most stars, including the Sun, will eventually become white dwarfs, which are dense remnants that continue to glow from residual heat but can no longer sustain nuclear fusion. In binary systems, white dwarfs can attract material from companion stars, leading to explosive events known as novas. More massive stars, however, end their lives in more dramatic fashion. After exhausting their core energy, they may explode in a supernova, leaving behind either a neutron star, a dense object comprised mostly of neutrons, or a black hole, an incredibly dense region where gravity prevents even light from escaping. The study of these stellar remnants provides crucial insights into stellar formation and the future of the universe, reflecting both the past processes that shaped celestial bodies and their potential impact on Earth.
Stellar Remnants
FIELDS OF STUDY: Astronomy; Astrophysics; Stellar Astronomy
ABSTRACT: As stars burn energy, they change, often dramatically. What is left behind after the final change is called a "stellar remnant." The type of remnant is determined by the nature of the original star. By studying stellar remnants, scientists can learn more about how stars are created, how long they last, and how they interact with other forces in space.
Life Cycles of Stars
Stars are not living things, but they do have a life cycle. They are created by the force of gravity acting on clouds of dust and stellar gases. The energy that sustains them is produced in their core, where various elements undergo constant nuclear fusion. As these elements are depleted, the stars change and eventually die. This cycle happens over the course of millions or even billions of years, so it is not possible to observe it directly. However, astronomers learn about the process by studying the remnants left behind by dying stars.
One of the ways scientists learn about stars is by studying the various kinds of electromagnetic (EM) radiation they emit. The visible light emitted by a star provides information about its temperature, with brighter stars typically being hotter. The presence of a considerable amount of ultraviolet light reveals that a star is very active and new. X-rays are associated with neutron stars and black holes, and white dwarfs emit both x-rays and ultraviolet (UV) radiation. Gamma rays, which have the shortest wavelength of all EM radiation, are emitted by both black holes and the supernovas that often precede them.
How Stars Evolve
Stars evolve in different ways, depending on their mass. Astronomers believe that most stars in the universe, including Earth’s sun, will eventually become white dwarfs. The core of a white dwarf is formed by fusing helium into carbon. White dwarfs do not get hot enough to burn carbon, so this core will contract until its electrons occupy the smallest space allowed by quantum mechanics. While this happens, the temperature of the core increases to about 100,000 kelvins. This residual heat allows the white dwarf to continue glowing, even though its light will be dim.
Sometimes, if a white dwarf is part of a binary system and its companion star is close enough to be affected by its gravity, the white dwarf will attract matter from the atmosphere of the companion star. When enough matter has accreted, the hot core of the white dwarf can cause it to ignite, creating an explosive flare called a nova. Astronomers once thought that novas were caused by the birth of new stars. It is now known that they are, in fact, signposts of dying white dwarfs.
When a star is more than eight times the mass of Earth’s sun, it ends its life in a different way. As a massive star’s core loses energy, it can no longer resist the force of its own gravity. The star collapses, then explodes in a supernova. Much of the star’s matter is lost in the explosion. The remaining core collapses once again, this time shrinking to such an extent that its protons and electrons are forced to combine and form neutrons. The result is a small, incredibly dense object called a neutron star.
However, if the core remaining after a supernova is massive enough—about twenty times the mass of the sun or greater—its gravity is so great that it simply does not stop collapsing. Eventually, all of the core’s mass is concentrated in a single point, known as a singularity. The region surrounding it is called a black hole.
Studying Stellar Remnants
Stellar remnants, such as white dwarfs, neutron stars, and black holes, offer astronomers a look into both the past and the future of the universe. Learning how stars change and die provides insight into star formation and composition, as well as how they may have affected Earth in the past and how they may affect it in the future.
PRINCIPAL TERMS
- black hole: a region of space with a gravitational pull so strong that even light cannot escape it.
- electromagnetic (EM) radiation: energy created by interactions between electrically charged particles and transmitted in the form of waves; includes the visible light spectrum, as well as radio waves, microwaves, infrared, ultraviolet, x-rays, and gamma rays.
- neutron star: an incredibly dense, rapidly spinning star, consisting mainly of closely compacted neutrons left behind after the gravitational collapse of a massive star.
- nova: a sudden, intense, ultimately short-lived increase in the brightness of a white dwarf star, caused by an accumulation of hydrogen that triggers a runaway thermonuclear reaction.
- supernova: the immensely forceful explosion produced when a massive star reaches the end of its life cycle and collapses under its own mass.
- white dwarf: a small, very dense star that has exhausted nearly all of its fuel and is nearing the end of its life cycle.
Bibliography
Bernoskie, Brandi, Heather Deiss, and Denise Miller. "What Is a Supernova?" NASA Education. NASA, 4 Sept. 2013. Web. 31 Mar. 2015.
"Black Holes." NASA Science.NASA, 30 Mar. 2015. Web. 31 Mar. 2015.
Frazier, Sarah. "Why the Sun Won't Become a Black Hole." National Aeronautics and Space Administration, 26 Sept. 2019, www.nasa.gov/image-feature/goddard/2019/why-the-sun-wont-become-a-black-hole. 14 June 2022.
Keeton, Charles. Principles of Astrophysics: Using Gravity and Stellar Physics to Explore the Cosmos. New York: Springer, 2014. Print.
Kornreich, Dave. "What Is a Nova?" Ask an Astronomer. Cornell U Astronomy, 24 Jan. 1999. Web. 31 Mar. 2015.
"Neutron Stars: Incomprehensible Density." Science and Space. Natl. Geographic Soc., n.d. Web. 31 Mar. 2015.
O’Neill, Ian. "Why Are Quark Stars So Strange?" Discovery News. Discovery Communications, 19 Jan. 2010. Web. 6 Apr. 2015.
"White Dwarf." Cosmos: The SAO Encyclopedia of Astronomy. Swinburne U of Technology, n.d. Web. 31 Mar. 2015.
"White Dwarfs: Aging Stars." Science and Space. Natl. Geographic Soc., n.d. Web. 31 Mar. 2015.